Pressure sensitive adhesive tapes or sheets comprising a base material formed of a polyolefin and a pressure sensitive adhesive layer formed thereon are known. These pressure sensitive adhesive tapes or sheets are usually wound up in the form of a coil to form rolls as products.
In these rolls, the pressure sensitive adhesive as an upper layer closely adheres to the back surface of the base layer (substrate) as a lower layer under relatively strong adhesion. It is hence difficult to unwind or peel off the wound tape or sheet for use as things are. Therefore, the back surface of the base layer has generally been subjected to a so-called releasability-imparting treatment so as to permit easy unwinding of the pressure sensitive adhesive tape or sheet without undergoing the migration to the opposite side (the back surface of the base layer) and blocking of the pressure sensitive adhesive, and the delamination of the base material upon its unwinding.
As such a releasability-imparting treatment, it has widely been used to date to apply a release agent to the back surface of a base layer in advance. It is however difficult to control contradictory properties that the release agent is caused to firmly adhere to the base layer, but made liable to suitably separate from the pressure sensitive adhesive layer. Therefore, it is necessary to use a specific release agent, to coat with a primer, or to be subjected to a corona discharge treatment prior to the coating of the release agent.
As exemplary release agents used generally, may be mentioned linear alkyl-containing polymer (for example, polyvinyl ester allylcarbamates), silicone polymers and perfluoro hydrocarbon polymers. These release agents cause the increase of the cost of materials and also the production cost because of the required complex coating process. .Moreover, even after producing the pressure sensitive adhesive tape or sheet, there is a problem in quality aspect that the release agent migrates to the pressure sensitive adhesive layer, thereby causing the reduction of its adhesion.
By the way, it is proposed in Japanese Patent Application Laid-Open No. 103975/1986 to simultaneously form and laminate a base layer and a pressure sensitive adhesive layer by a co-extrusion technique though it is in a surface protective film. According to this method, the process is simplified and the adhesion properties between the base layer and the pressure sensitive adhesive are improved. It is also paid attention as an excellent technique from the viewpoint of quality. However, in this case, it is also necessary to subject the back surface of the base layer to a releasability-imparting treatment in order to impart good unwinding ability to the protective film, because of the strong adhesion of the pressure sensitive adhesive to the base layer. The co-extrusion technique however fails to subject the back surface of the base layer to the releasability-imparting treatment in advance. This technique is hence accompanied by a more serious problem in that the restriction of season is imposed on the releasability-imparting treatment.